Perfect timing

NOW there's no excuse for being more than a million-billionth of a second late. Researchers at the US government's National Institute of Standards and Technology in Boulder, Colorado, have created a "femtosecond" clock so accurate that it would be no more than a second out after 30 billion years.

The clock counts the oscillations of a laser beam, which are held at an exact frequency by linking the laser to electrons jumping between energy levels in a mercury ion. These transitions tune the laser oscillator to "tick" 100,000 times as fast as conventional atomic clocks, which are based on caesium atoms. The faster tick improves timekeeping.

The researchers, whose invention will be described in a future issue of Science, expect it to improve navigation by satellite, which relies on accurate time measurements.

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